I The notion of locality in (Quantum) Physics should be clearly defined

[gentzen]

For me that's a clear contradiction of the very footnote they make "for clarity" in the very beginning. So for me the paper does not discuss adequately the precise meaning of "locality/non-locality" in the relativistic context.

Please provide a reference to a paper which adequately discusses that stuff for you.

 

[Morbert]

There's still a question of how the unitary evolution becomes non-unitary upon measurement, making the Born rule necessary. Saying it's inherently random is to make the stochastic part of QM a brute fact of nature.

If we model the microscopic system + apparatus + environment as an isolated system*, we can evolve this state unitarily and reduced states stochastically. There is no need to specify an objective moment of measurement where unitary evolution gives way to stochastic evolution, even if we are instrumentalists.

* (ignoring issues of environments with black holes and non-foliable spacetime etc)

 

[Lord Jestocost]

Just look at section A. The key points are clearly stated:
The point, however, is that in (3) they tacitly assumed not only "locality" but also realism, and that's leads to the Bell inequalities, which distinguishes all "local, realistic theories" from "non-realistic theories".

Unfortunately they do not discuss microcausality at all, except in this footnote, where they explicitly say that they do not take it into account, which is strange, because that's the very way, "locality" is mathematically implemented into the only viable relativistic QT we have today, i.e., relativistic, local QFT.

On the other hand in discussing Eq. (3) they state:For me that's a clear contradiction of the very footnote they make "for clarity" in the very beginning. So for me the paper does not discuss adequately the precise meaning of "locality/non-locality" in the relativistic context.

At this point, I agree with you, something similar to the way Żukowski and Brukner put it in “Quantum non-locality – it ainʼt necessarily so...”/1/:

“The terms ‘nonlocality’ or ‘quantum non-locality’ are buzzwords in foundations of quantum mechanics and quantum information. Most of scientists treat these terms as a more handy expression equivalent to the clumsy “violation of Bell’s inequalities”. Unfortunately, some treat them seriously. Even more unfortunately Bell himself used such terms in later works [1, 2] [26].”

/1/ Marek Żukowski and Časlav Brukner 2014 J. Phys. A: Math. Theor. 47 424009

 

[Simple question]

No, the same issue I described is still there.

I don't understand the contradiction. QM is linear and deterministic, and deals in probabilities (is stochastic).

That, again, depends on which interpretation you adopt. Some interpretations (such as the MWI and "objective collapse" interpretations) have this.

I was under the impression that to make QM "inherently random", objective collapse added a piece of math (representing the collapse), and thus was not mere interpretation.

But that does not mean QM interpretations cannot make claims about individual events or link them to the math. As noted above, many interpretations do that.

I agree. But the claims made by @vanhees71 in this thread are unsubstantiated and often wrong. Like post #49 "It's all due to local interactions between the electromagnetic field (laser) and the particles building up the BBO crystal." which is actually saying QFT is a realistic theory ... and there is no measurement problem.

Experiment and Bell's showed that QM is correct, thus Nature have spooky behaviors (like non-local and even a-causal correlations). There is no way to "interpret this out" using QM.

I don't expect either for him to apply his own interpretation to the experiment proposed in #71

What non-local constraint are you referring to?

Microcausality in QFT : "...must commute at space-like separated arguments.."
I assume that this is a critical feature of the theory that allows it to make correct predictions (has far as it is able to, within its well defined limit).

 

[PeterDonis]

QM is linear and deterministic, and deals in probabilities (is stochastic).

These statements contradict each other; "linear and deterministic" is inconsistent with "stochastic".

I was under the impression that to make QM "inherently random", objective collapse added a piece of math (representing the collapse), and thus was not mere interpretation.

Objective collapse interpretations do not add collapse to the math; that is already there in the basic math (for example, the PF 7 Basic Rules). Objective collapse interpretations make a claim about that piece of math, that it reflects an actual physical thing happening. Other interpretations do not make that claim.

But the claims made by @vanhees71 in this thread are unsubstantiated and often wrong.

This is way too strong. Differences of opinion about interpretations do not justify claims like "unsubstantiated and wrong". (There is a sticky thread at the top of the interpretations forum that contains forum guidelines; one of them relates to that very point.)

 

[PeterDonis]

Microcausality in QFT : "...must commute at space-like separated arguments.."

Ah, ok. But you seem to be thinking that this feature is somehow unique to QFT and raises issues. Actually, that's not the case. Commuting measurements where different observables are being measured are the norm--the natural case. Classical measurements of different observables always commute. The new feature that QM (and QFT, which just makes clearer how relativity plays into it all) introduces is non-commuting measurements--measurements of different observables (like spin-z and spin-x) that do not commute. The QFT constraint just makes clear that the domain in which such non-commuting measurements are possible is restricted by the requirements of relativistic causality.

I assume that this is a critical feature of the theory that allows it to make correct predictions

To the extent that this is the case, as above, the predictions are the same as those in non-relativistic QM, and indeed in classical physics.

 

[vanhees71]

I don't understand the contradiction. QM is linear and deterministic, and deals in probabilities (is stochastic).

QM is described by usual partial differential equations like the Schrödinger equation for the wave function. It's not a stochastic differential equation, or are you referring to the quantum Langevin Schrödinger equation, which is one way to describe an open quantum system?

The meaning of the state is probabilistic. The time evolution of usual QM for closed systems is described by a usual partial differential equation, not a stochastic one.

I was under the impression that to make QM "inherently random", objective collapse added a piece of math (representing the collapse), and thus was not mere interpretation.

That's true. There are attempts to extend the quantum formalism with some stochastic collapse mechanism, but that's not QM anymore but a new theory. There's, however, not the slightest hint that such an alteration is needed anywhere.

I agree. But the claims made by @vanhees71 in this thread are unsubstantiated and often wrong. Like post #49 "It's all due to local interactions between the electromagnetic field (laser) and the particles building up the BBO crystal." which is actually saying QFT is a realistic theory ... and there is no measurement problem.

Don't interpret something into what I'm saying, which I never said. QFT as any QT is not realistic, i.e., within this theory not all observables always take determined values. Also it's weird to claim that standard local relativsitic QFT were wrong, while in fact it's the most successful class of theories ever discovered!

Experiment and Bell's showed that QM is correct, thus Nature have spooky behaviors (like non-local and even a-causal correlations). There is no way to "interpret this out" using QM.

Experiment shows that also local relativistic QFT is correct, and this excludes spooky actions at a distance by construction. That's a mathematical property of the theory and cannot be argued away by some "interpretation" gibberish.

I don't expect either for him to apply his own interpretation to the experiment proposed in #71Microcausality in QFT : "...must commute at space-like separated arguments.."
I assume that this is a critical feature of the theory that allows it to make correct predictions (has far as it is able to, within its well defined limit).

It's among THE key features, and it predicts from the start very well established facts about nature like the CPT symmetry and the relation between spin and statistics.

I've no clue what you mean in #71. Whether I use one equipment to run an experiment 10000 times or whether I build 10000 different equipments doesn't make any difference. It's just preparing large enough ensembles to have a high significance in my statistical tests of the probabilistic predictions of Q(F)T.

 

[Lord Jestocost]

I don't understand the contradiction. QM is linear and deterministic, and deals in probabilities (is stochastic).I was under the impression that to make QM "inherently random", objective collapse added a piece of math (representing the collapse), and thus was not mere interpretation.I agree. But the claims made by @vanhees71 in this thread are unsubstantiated and often wrong. Like post #49 "It's all due to local interactions between the electromagnetic field (laser) and the particles building up the BBO crystal." which is actually saying QFT is a realistic theory ... and there is no measurement problem.

Experiment and Bell's showed that QM is correct, thus Nature have spooky behaviors (like non-local and even a-causal correlations). There is no way to "interpret this out" using QM.

I don't expect either for him to apply his own interpretation to the experiment proposed in #71Microcausality in QFT : "...must commute at space-like separated arguments.."
I assume that this is a critical feature of the theory that allows it to make correct predictions (has far as it is able to, within its well defined limit).

Ĉaslav Brukner and Anton Zeilinger in “Information and fundamental elements of the structure of quantum theory”/1/:

“10 MEASUREMENT - THE UPDATE OF INFORMATION

In this section, it will be argued that identifying the quantum state of a system with the catalog of our knowledge of the system leads to the resolution of many of the seemingly paradoxical features of quantum mechanics connected to the so-called measurement problem.

In a quantum measurement, we find the system to be in one of the eigenstates of the observable defined by the measurement apparatus. A specific example is the case when we are considering a wave packet as being composed of a superposition of plane waves. Such a wave packet is more or less well-localized, but we can always perform a position measurement on a wave packet which is better localized than the dimension of the packet itself. This, sometimes called “reduction of the wave packet” or “collapse of the wave function”, can only be seen as a ”measurement paradox” if one views this change of the quantum state as a real physical process. In the extreme case it is often even related to an instant collapse of some physical wave in space.

There is no basis for any such assumption. In contrast, there is never a paradox if we realize that the wave function is just an encoded mathematical representation of our knowledge of the system. When the state of a quantum system has a non-zero value at some position in space at some particular time, it does not mean that the system is physically present at that point, but only that our knowledge (or lack of knowledge) of the system allows the particle the possibility of being present at that point at that instant.

What can be more natural than to change the representation of our knowledge if we gain new knowledge from a measurement performed on the system? When a measurement is performed, our knowledge of the system changes, and therefore its representation, the quantum state, also changes. In agreement with the new knowledge, it instantaneously changes all its components, even those which describe our knowledge in the regions of space quite distant from the site of the measurement. Then no need whatsoever arises to allude to notions like superluminal or instantaneous transmission of information.”

/1/ in “Time, Quantum and Information” (A collection of research papers written in commemoration of the 90th birthday of C. F. von Weizsäcker), eds. Lutz Castell and Otfried Ischebeck, 2003

 

[vanhees71]

Indeed, Brukner and Zeilinger just set the record straight by using the minimal statistical information. That solves all pseudo-problems. The only problem that remains is that some philosophers cannot accept that Nature behaves as she does and not as they want. They are still confined in their "classical worldview". That's all that's left.

 

[Lord Jestocost]

.... The only problem that remains is that some philosophers cannot accept that Nature behaves as she does and not as they want. They are still confined in their "classical worldview".

But not, for example, C. F. von Weizsäcker!

 

[vanhees71]

I've no clue, what C. F. von Weizsäcker was after ;-).

 

[Lord Jestocost]

I've no clue, what C. F. von Weizsäcker was after ;-).

Maybe, the following might be of help.

Klaus Michael Meyer-Abich in „Science and Its Relation to Nature in C.F. von Weizsäcker’s Natural Philosophy”/1/:

“True enough, Weizsäcker considered the concepts of science as 'completely dark and devoid of explanation' [14, p. 287}. This is not the way physicists generally feel but the fact that they can handle these concepts does indeed not explain what happens when they do so. Any physicist approached by Weizsäcker with the Socratic question as to whether he understands what he is doing would soon have to admit his ignorance. So far this has been the distinction between a philosopher and a physicist.”

/1/ in “Time, Quantum and Information” (A collection of research papers written in commemoration of the 90th birthday of C. F. von Weizsäcker), eds. Lutz Castell and Otfried Ischebeck, 2003

 

[Morbert]

In contrast, there is never a paradox if we realize that the wave function is just an encoded mathematical representation of our knowledge of the system. When the state of a quantum system has a non-zero value at some position in space at some particular time, it does not mean that the system is physically present at that point, but only that our knowledge (or lack of knowledge) of the system allows the particle the possibility of being present at that point at that instant.

If the knowledge is merely knowledge of a possible location of the particle, then we have conceptualised a property of the particle independent from a measurement context. To avoid the charge of introducing hidden variables (see common critiques of psi-epistemic interpretations), the knowledge represented by the state has to be re-examined. Two approaches:

i) The knowledge is knowledge of possible instrument responses if a measurement is carried out. We don't say "there is a probability p the particle is in some interval X". We say there is a probability p that a detector tuned to some interval X will click.

ii) Develop a system of logic for properties of microscopic systems that accommodates the complementary character of quantum theories (see decoherent histories).

 

[Simple question]

Ah, ok. But you seem to be thinking that this feature is somehow unique to QFT and raises issues.

Not at all, I am fine with any constraints and rules that helps in any model. I see no issue with micro-causality. But there is a specific person on this forum that clearly does not understand its domain of application, and makes wrong claims about its domain of application.

Actually, that's not the case. Commuting measurements where different observables are being measured are the norm--the natural case. Classical measurements of different observables always commute. The new feature that QM (and QFT, which just makes clearer how relativity plays into it all) introduces is non-commuting measurements--measurements of different observables (like spin-z and spin-x) that do not commute.

Thank you for this summary. It's actually one of the best I've read.

The QFT constraint just makes clear that the domain in which such non-commuting measurements are possible is restricted by the requirements of relativistic causality.

But surely you don't infer that this restriction means those measurement are not possible. It mean those measurement are not in the domain of QFT.
Yet @vanhees71 thinks that the theory comes first and that implies that those experiment results (since Aspect, 40 years ago !!!) should be dismissed. I cannot think of more anti-scientific stance. This is not at all an dispute about interpretations.

To the extent that this is the case, as above, the predictions are the same as those in non-relativistic QM, and indeed in classical physics.

Maybe it is, so I'll assume micro-causality never enter the equation to predict entanglements results, or "the speed at which" collapse occurs ... if there is such a thing.[/B]

 

[PeterDonis]

there is a specific person on this forum that clearly does not understand its domain of application, and makes wrong claims about its domain of application

Again, this is way too strong and you need to stop making this claim.

surely you don't infer that this restriction means those measurement are not possible.

Of course not. Nobody is claiming that.

It mean those measurement are not in the domain of QFT.

It means no such thing. Commuting measurements are just as much in the domain of QFT as non-commuting measurements.

@vanhees71 thinks that the theory comes first and that implies that those experiment results (since Aspect, 40 years ago !!!) should be dismissed.

I do not see @vanhees71 making any such claim. You are seriously misinterpreting his posts if you think this is what he is saying.

I'll assume micro-causality never enter the equation to predict entanglements results

You cannot assume any such thing. All you can assume is what I explicitly said: that for the case of spacelike separated measurements, the predictions of QFT are the same as those of non-relativistic QM. (It is true that for the case where such measurements are made on entangled particles, the predictions will not be the same as those of classical physics, since the latter will never predict violations of the Bell inequalities. I did not intend to state otherwise, but on reading my previous post I see how it could have been interpreted that way. Sorry for the ambiguity on my part.)

"the speed at which" collapse occurs

There is no such thing except in "objective collapse" interpretations, which, as I think has already been noted in this thread, actually become different theories (i.e., different math from the standard math of QM, making different experimental predictions) when developed fully.

 

[Simple question]

QM is described by usual partial differential equations like the Schrödinger equation for the wave function. It's not a stochastic differential equation

Nice ! You finally walked-back your claim that "nature is fundamentally random". Because if the theory is not, you then have no mathematical way to prove it.

The meaning of the state is probabilistic.

So is game theory or statistical mechanics. Still, it does not make those "fundamentally random" nor "mysterious".

That's true. There are attempts to extend the quantum formalism with some stochastic collapse mechanism, but that's not QM anymore but a new theory. There's, however, not the slightest hint that such an alteration is needed anywhere.

You yourself provided those hints. You are just stuck in the past and you cannot accept that Nature behaves as she does and not as you want.

Don't interpret something into what I'm saying, which I never said. QFT as any QT is not realistic, i.e., within this theory not all observables always take determined values.

Everyone knows that QFT is unrealistic it has been mathematically proven. You just don't understand what that means, which is: you cannot make claim about NATURE using it. You can barely describe the most basic setup with it (see **)

Also it's weird to claim that standard local relativsitic QFT were wrong

Another falsehood. You would be hard pressed to quote any such "claim" from anyone on this thread.
You are wrong, not the theory. You seem to identify yourself with it. That is weird.

while in fact it's the most successful class of theories ever discovered!

"class ?" "successful ?". Unsubstantiated opinion holds no water on scientific forum.

Experiment shows that also local relativistic QFT is correct

Experiment within its domain only.

and this excludes spooky actions at a distance by construction.

No it does not. Experiment show that spooky correlation at a distance exist. So the theory does not "exclude" that, because it would mean the theory is wrong or incomplete.
Science prefer experiment over theory, by construction

That's a mathematical property of the theory and cannot be argued away by some "interpretation" gibberish.

So stop that gibberish. Mathematical property are just that, not fact about Nature.

It's among THE key features, and it predicts from the start very well established facts about nature like the CPT symmetry and the relation between spin and statistics.

Cool. So micro-causality has its purpose. I am really not surprised. So how to use it in the simplest setup (see **) ? or in entanglement cases, swapping maybe ?

I've no clue what you mean in #71. Whether I use one equipment to run an experiment 10000 times or whether I build 10000 different equipments doesn't make any difference. It's just preparing large enough ensembles to have a high significance in my statistical tests of the probabilistic predictions of Q(F)T.

** It doesn't surprise me. You are not interested by experiments nor how those can be described by a theory. Not even in principle. You wrote:

The detectors don't negotiate anything. It's just the interaction of the photon with the material around it. Where it will be detected is random,

It's just a contradiction. Those 10000 labs are "prepared" in different light cone. Your own idiosyncratic miss-use of QFT explicitly forfeit its predictive power because you assume micro-causality and local interaction apply. Always everywhere.
All of these events are space like, so what say you ?
Hint: You should have stuck with the minimal (not gibberish) interpretation which is:

Natural science is not for explaining the world, and especially not describing at best as possible with mathematical models. You don't aim to describe nature because it is un-real. And just shut-up and calculate probabilities, even though nature deal in events, not probability of platonic ensemble.

And if one want to shoot only one photon some place, or entangle of few QBit in a QComputer, it is not worthy of "science". The least is the best it can do.

 

[Simple question]

You cannot assume any such thing. All you can assume is what I explicitly said: that for the case of spacelike separated measurements, the predictions of QFT are the same as those of non-relativistic QM.

But that is not the point ! I agree with that perfectly: QFT have no additional claim to make about entanglement of spacelike measurement. @vanhees71 is not agreeing with this. So why do you say *I* made too strong claim ?

The ambiguity is probably on my side because if anything I said that QFT as no claim (note: not contradictory claim) to make about Bod and Alice's local mater and field. They are space-like, so my error is probably to think entanglement swapping involve non-commuting observable.

Anyway the point is that only @vanhees71 bring up micro causality as the only way to (and with godly precision) to describe this, while this has no bearing on Bell's locality tests. How could this be an "interpretation issue" instead of "a plain mistake ?"

There is no such thing except in "objective collapse" interpretations, which, as I think has already been noted in this thread, actually become different theories (i.e., different math from the standard math of QM, making different experimental predictions) when developed fully.

I though so. I doubt I would ever be able to understand how this would explain entanglement "speed". But that's for another thread.

 

[gentzen]

and this excludes spooky actions at a distance by construction.

No it does not. Experiment show that spooky correlation at a distance exist. So the theory does not "exclude" that, because it would mean the theory is wrong or incomplete.
Science prefer experiment over theory, by construction

Because "spooky action at a distance" (or the German equivalent) was coined by Einstein, its meaning is much more technical and fixed, than you seem to assume. This meaning simply does not include nonlocal correlations. (If you want, include the Bohmian type of nonlocality in it, and all the theories/interpretations with an explicit collapse of the wavefunction, but mere "correlations" are too "passive" as to be termed "action" by Einstein.)

"class ?" "successful ?". Unsubstantiated opinion holds no water on scientific forum.

So stop that gibberish. Mathematical property are just that, not fact about Nature.

I get the impression that you are trolling. But maybe my impression is wrong, and this is just your way to have a lively discussion.

 

[PeterDonis]

Thread closed for moderation.

 

[PeterDonis]

Natural science is not for explaining the world

An off topic subthread that was more or less spawned by this comment has been deleted. Please keep discussion in this thread focused on the specific topic of how "locality" is defined in QM. More general discussion of what science is "for" and how it should be done belongs in a separate thread in General Discussion if anyone wants to pursue it.

 

[PeterDonis]

Thread reopened.

 

[PeterDonis]

Brukner and Zeilinger just set the record straight by using the minimal statistical information. That solves all pseudo-problems.

This claim, of course, requires that you adopt the particular interpretation you describe. It should be obvious to you by now that not everyone accepts that interpretation. And the guidelines for this forum make clear that no particular interpretation can be asserted to be "correct". Everyone in the discussion must accept that there are different interpretations of QM that say different, sometimes incompatible things, and that that fact is not going to change as a result of any discussion here.

The only problem that remains is that some philosophers cannot accept that Nature behaves as she does and not as they want. They are still confined in their "classical worldview". That's all that's left.

This is the kind of claim that the guidelines for this forum do not permit. The fact that you prefer a particular interpretation does not allow you to claim that anyone who doesn't accept it is not accepting how Nature behaves. The only things we know about how Nature behaves are the things we see in experiments, and all QM interpretations agree on all experimental predictions.

 

[Demystifier]

A comment on the title of this thread. It's not sufficient that locality (or anything else for that matter) is defined clearly. It also must be defined adequately. The standard QFT definition is clear, but in the context of quantum foundations it is not adequate.

The standard QFT definition of locality talks about observables, not about events (see my first post in this thread). For practical purposes that's perfectly OK, but in quantum foundations one goes beyond pure practicality and tries to understand how exactly the events happen. For that purpose one needs a definition of locality that more directly deals with events, which is why the standard QFT definition is not adequate.

 

[Demystifier]

realistic (which means that all observables always take determined values).

This is an utter nonsense, at least 3 levels.

1. Observable is a self-adjoint operator, an operator cannot take a value (if by "value" one means a number). Operator is a map from a Hilbert space to itself. It can be represented by a matrix. But it cannot be represented by a number, it cannot "take a value".

2. It is not clear what "determined" means. Deterministic, as opposed to stochastic/probabilistic? If so, then it's not what realistic means. Or maybe definite, meaning defined (even when it's not measured)? Yes, that would be a better explanation of realistic, but then it should be said so.

3. The Bell theorem assumes that some variables, which he calls beables, take definite values. These variables may (or may not) have a superficial similarity with some observables. For example, in Bohmian mechanics, the beables are particle positions, which have a superficial similarity with position observables. But the point is that it only refers to some observables, not all observables. For most observables, there are no corresponding beables.

 

[weirdoguy]

Observable is a self-adjoint operator

Some people (e.g. Ballentine, if I recall correctly) differentiate between observables as quantities that can be measured in an experiment, and operators that are associated with them.

 

[Demystifier]

Some people (e.g. Ballentine, if I recall correctly) differentiate between observables as quantities that can be measured in an experiment, and operators that are associated with them.

Yes, but then the post by @vanhees71 would make even less sense, because then "observables" would always commute, not only at spacelike separations.

 

[martinbn]

Some people (e.g. Ballentine, if I recall correctly) differentiate between observables as quantities that can be measured in an experiment, and operators that are associated with them.

I think everyone does. Demystifier just continues with his sophistry.

 

[weirdoguy]

because then "observables" would always commute, not only at spacelike separations.

How do you define commutator of two quantities?

 

[vanhees71]

Yes, but then the post by @vanhees71 would make even less sense, because then "observables" would always commute, not only at spacelike separations.

Why that? Of course there are no self-adjoint operators in the lab nor Hilbert spaces and all that. That's the mathematical description. In the lab you have accelerators, detectors, lasers, and all that theoreticians don't want to get their hands dirty with ;-).

 

[Demystifier]

How do you define commutator of two quantities?

As the commutator under multiplication, $AB-BA$. I guess I don't need to explain what is multiplication of operators, and what is multiplications of real and complex numbers.